1. Field of the Invention
The present invention is directed to a computer tomography apparatus, and in particular to such an apparatus including means for identifying a change in the focus position of the x-ray beam and means for taking this change into account in constructing the image for display.
2. Description of the Prior Art
Computer tomography devices are known which generally include a measuring unit having an x-ray source and a radiation receiver. The x-ray source emits a fan-shaped radiation beam in which a patient is disposed, with the radiation attenuated by the patient being incident on a radiation receiver consisting of a row of individual detectors. Each detector forms an electrical signal corresponding to the radiation intensity incident thereon. The x-ray source and radiation detector are rotated around the patient for transirradiating a measuring field from different directions. The output signals from the individual detectors at the different transirradiation directions are supplied to a computer which constructs an image therefrom for visual display.
In computer tomography devices of this type, a condition for obtaining an image free of artifacts is that the focus be at a predetermined known position with respect to the radiation receiver. In practice, however, the focus will migrate or change position slightly relative to the known or rated position, particularly in the longitudinal direction of the patient support (z-direction). This is primarily due to heating of the x-ray tube, in particular the anode thereof, during use.
One proposed solution to compensate for such changes in focus position is described in U.S. Pat. No. 4,559,639, wherein a plurality of detectors are provided to measure the change in focus position by direct sensing thereof.